(a) Field of the Invention
The present invention relates to the field of geophysics, and is particularly concerned with methods for detecting the contemporary geodynamic movements or more properly the geodynamics in a massif. The invention is of particular advantage in the case of geological surveying for construction. It may be also useful in solving scientific and practical problems arising in the field of mining, seismology, and prospecting for and survey of minerals, by using the methods of structural-geodynamic mapping.
(b) Prior Art
One of the main factors capable of decreasing a geological risk in designing and building up-to-date engineering structures (especially unique ones) is an availability of structural-geodynamic maps containing information both on the tectonic structure of the area being surveyed and on the nature and the rate of the current geodynamic processes. Such information is of particular value for the regions with a higher seismic activity, in the areas susceptible to formation of cock-pits, landslip, or having active tectonic structures, and in the areas where the rock mass is intensively influenced by the presence of engineering structures.
The structural mapping is based on the data obtained by drilling. However, the drilling operations involve considerable expenses and cannot be directly used for obtaining information on the nature of the contemporary geodynamic movements.
Conventional geophysic methods of structural mapping are based on analyzing characteristics of the natural and artificially established fields. The presence and nature of disturbances occuring in such fields allow judging tectonic faults and the attitude thereof, and provides a basis for making structural maps. In principle, all geophysic fields contain information on the contemporary geodynamic movements. However, obtaining this information in a usable form is practically impossible because of the disturbances caused in the fields being examined by natural inhomogeneity of rock mass (which inhomogeneity creates complications in carrying out other works utilizing geophysic methods, such as structural mapping, prospecting for and surveying minerals). For instance, in practicing widely known methods of structural mapping based on establishing electromagnetic fields (cf. A. B. Velikin, G. S. Frantov "Electromagnityne polia, primenjaemye v indukzionnyh metodah electromagnitnoi razvedki", Leningrad, 1962 pp. 221-212 and 260) a deep sounding of rock medium is often needed, wherein the electromagnetic field reaches a fault being searched (tectonic dislocation) which is covered by overlying deposits, and the intensity of the field is sufficient enough to provide a recording disturbance caused by said tectonic fault.
In the case of such sounding the whole stratum of overlying rock above the tectonic dislocation screens the latter and the electromagnetic field being recorded is indicative of all the non-uniformities existing in said stratum, and it is not unusual when the disturbances caused by these non-uniformities are by their intensity and morphology similar to those caused in the field by the tectonic dislocation itself.
A circular sounding which is used for structural mapping, and prospecting for minerals produces essentially the same results. The circular sounding is a modification of the deep sounding, wherein the electromagnetic field set up in a massif is examined from different azimuth directions from the source of electromagnetic radiation at a distance measuring tens of meters (cf. V. J. Hmelevskoi "Osnovnoi kurs electrorazvedki", ch.II, izd. Moskovskogo universiteta, 1971, p.256).
The area to be explored having such a radius includes inhomogeneous and non-isotropic rocks having a long time of relaxation (many decades), which makes this method unsuitable for detecting the contemporary geodynamic movements. The depth to which a survey is carried out makes it probable that the medium being explored includes magnetic inclusions, which considerably complicates the measuring techniques.
Screening the tectonic dislocation by the stratum of overlying rocks similar to those described above also takes place in the case of examining natural fields of forces (gravitational, magnetic, gas and other fields) in geophysic methods of survey.
The information on the character of the contemporary geodynamic movements can be obtained by conventional instrumental survey methods such as precision repeated levelling, clinometering, deformography, extensometering, etc.). All these methods, however, are local and require complex and costly measures to eliminate the influence of such destabilizing factors as temperature, humidity, pressure, etc. on the measurement results, which lowers the promptness of these methods.
It should be noted that all the above-mentioned methods, when combined, may be used for making structural geodynamic maps, but for practical needs when detailed information is required about relatively large areas such mapping involves tremendous expenses.
The first attempt to provide an economically viable solution for the above problem was development of a method of detecting the contemporary geodynamic movements of tectonic structures utilizing an emanation survey (cf. USSR Author's Certificate No. 396,659, Int.Cl G 01V5/00).
This method utilizes an emanation ability of overlying deposits and comprises selecting, with the aid of a gas sampler and vacuum pump, samples of soil air from the wells up to 0.8 m deep drilled at the profile stations, and determining with the aid of analyzing devices the concentration and composition of the radioactive gases (radon and thoron). The changes in concentration of said gases relative to the background concentration is used to judge the contemporary geodynamic movements of the tectonic structures.
The use of the prior art emanation survey is based on the experimentally established fact of interrelation in space and time of emanation anomalies with the areas of the contemporary geodynamic movements.
The above method does not require costly and power-consuming equipment, and its advantage over the prior art methods lies in the promptness of the surveying operation, which makes it suitable for examining rapidly progressing dynamic processes and/or dynamic processes finely differentiated in space, which practically were not possible to examine.
The necessity to introduce equipment (gas sampler) into the medium being examined leads to a higher labour consumption (2 to 3 men are required to carry out such measurements and hence to a higher cost of survey operation, and also may cause a damage of pavements (on the roads and side-walks). When such surveys are carried out in towns and cities special measures must be taken to prevent the damage of service lines (cables, water and gas pipe lines, etc.). Furthermore, carying out this method involves earth work which considerably lowers the promptness of carrying out this method.
Since the above method is based on analysing the natural field, the characteristics of this field cannot be arbitrarily changed. In particular, the region of surveying medium by using radon which is capable of providing more information than the second gas (i.e. thoron) represents a lower half-space of about 10 m radius, and therefore the depth of survey cannot be adjusted to another value corresponding to a depth of the medium position. This leads to the distortion of the measurement results, caused by superposition of the emanation field variation due to inhomogeneity of the medium being surveyed.
The reliability of the data thus obtained is also lowered to a great extent by the influence that baro-hydrothermal conditions of the rocks exert on the emanation survey results, as well as by their initial condition (for instance loosening of soil as a result of agricultural work).
A substantial disadvantage of this method lies in that the physical characteristic of the medium being examined, selected for measurements, has a low information ability: the variation of this characteristic is used to form a judgement on variation of the contemporary tectonic stress fields, and on the geodynamic movements in the massif corresponding to these variations. Laboratory research carried out by Moscow geological survey institute has shown that emanation ability of soil is determined not only by the rate of the contemporary movements but also by the dynamics of these movements in time. It has been established that even insignificant but rapidly changing movement sharply increases the emanation ability of soil, and at the same time intensive but slowly changing movement may under certain conditions even decrease such ability. Therefore, the rate of variation of emanation ability of soil, determined by repeated measurements, makes it possible to determine one of the qualitative characteristics of the contemporary movements, namely, rate of their variation (dynamics) in time relative a certain terrain (since such rate of variation manifest itself most intensively over tectonic disturbances and the areas influenced thereby), but does not permit quantitative measurement of the intensity of the detected movements.
The information obtained by measuring emanation ability of soil, as described above, is of a scalar character, that is variation of the emanation ability has a scalar character, which variation is detected as a result of repeated measurements. Therefore, this method does not allow determining the character of the deformation caused by the detected geodynamic processes, thus rendering it less capable of providing more information.
In addition, the field of application of the above method is limited by that measuring the emanation of the soil gases may be effected only if the overlying soils are those like clay, loam, or black soil (chernozem).